ADC Pi

The ADC Pi is an 8 channel 17 bit analogue to digital converter designed to work with the Raspberry Pi platform. The ADC Pi is based on two Microchip MCP3424 A/D converters each containing 4 analogue inputs with up to 18 bit resolution. The MCP3424 is a delta-sigma A/D converter with low noise differential inputs.

Introduction:

The ADC Pi is pin compatible with the the Raspberry Pi model B+.

We designed the ADC Pi to work as a single ended A/D converter using the internal 2.048V reference voltage with the -V pins tied to ground. A voltage divider on the ADC Pi board brings the input voltage range to a much more useful 0 – 5.06V. In this configuration the sample size is 17 bits for each channel.

The ADC Pi is powered through the host Raspberry Pi using the GPIO port and extended pins on the GPIO connector allow you to stack the ADC Pi along with other expansion boards.

The two MCP3424 A/D converters communicate via i2c to the host Raspberry Pi giving you eight analogue inputs to use. A logic level converter is included on the ADC Pi board giving you a buffered 5V i2c port making it easy to add other I2C devices which operate at 5 volts without damaging the raspberry pi 3.3 volt i2c port. The i2c buffer uses N-channel mosfets with a maximum drain current of 100mA.

Datasheets for the MCP3424 can be found on the technical information tab.

The I2C address bits are selectable using the on-board jumpers. The MCP3424 supports up to 8 different I2C addresses so with two A/D converters on each ADC Pi you can stack up to 4 ADC Pi boards on a single Raspberry Pi giving you 32 ADC inputs.

The MCP3424 contains an on-board 2.048V reference voltage with an input range of ±2.048V differentially (full scale range of 4.096V/PGA). A programmable Gain Amplifier gives the user a selectable gain of x1, x2, x4 or x8 before the analogue to digital conversion takes place.

The data rate for analogue to digital conversions is 3.75 (17 bit), 15 (15 bit), 60 (13 bit) or 240 (11 bit) samples per second. Data rate and resolution can be configured within software using the I2C interface.

Unused inputs should be tied to ground.

I2C Address Selection

The MCP3424 analogue to digital converter contains two address select pins which can be tied to Vss, Vdd or left floating. This gives 8 possible I2C addresses for each chip. The ADC Pi V2 contains two MCP3424 chips so you can stack up to 4 ADC Pi V2 boards on a single Raspberry Pi. To simplify address selection on the ADC Pi V2 we have included a set of address selection pins which can be configured using the included jumpers. The illustrations below show the four recommended configurations for your ADC Pi V2 and the associated I2C addresses.

Note:
Disconnect the ADC Pi V2 from the Raspberry Pi before changing the address pins. You may need to short the 5V and ground with a resistor to discharge the capacitors in order for the new addresses to be recognised.

I2C Address Table

Adr 0

Adr 1

I2C Address

Low or Float

Low or Float

0x68

Low

Float

0x69

Low

High

0x6A

Float

Low

0x6B

High

Low

0x6C

High

Float

0x6D

High

High

0x6E

Float

High

0x6F

Warning

Do not under any circumstanced connect the two centre pins together. This will create a direct short between the 5V and ground pins and will damage or destroy your Raspberry Pi and ADC Pi Plus board.